Surface-protective films for metallic products or plastic products have hitherto been used in such a manner that the films are applied to surfaces of these products for the purpose of protecting the products against marring, soiling, or corrosion during transportation, storage, or processing and peeled therefrom after the temporary protection. The surface-protective films are hence required to be adhered to the metallic products or plastic products during the transportation, storage, or processing thereof and to be capable of being easily peeled from the adherends when the products are ready for use.
However, the pressure-sensitive adhesive sheets for use as such surface-protective sheets generally undergo an increase in adhesive force after application because the wetting of the pressure-sensitive adhesive proceeds with the lapse of time. The surface-protective films hence become difficult to peel from the adherends, and there are cases where the peeling operation requires much time or leaves a residue of the pressure-sensitive adhesive on the adherends. Especially in application to adherends which have undergone a surface treatment, such as painted steel sheets, the wetting of the pressure-sensitive adhesive is apt to proceed due to the surface roughness of the adherends and the adhesive force is hence apt to increase. Furthermore, when such members covered with surface-protective films are placed outdoors during execution of works and the protective films are peeled therefrom after the execution, there are often cases where the base material or adhesive deteriorates to leave an adhesive residue and this impairs the appearance of the products.
A technique generally employed for preventing such a phenomenon is to impart weatherability to the base material or adhesive layer. However, especially when a weathering agent is incorporated into an adhesive, the viscosity of the adhesive often increases considerably depending on the constitution of the adhesive components or on the kind of the weathering agent, making the application thereof impossible. Namely, there are cases where productivity is impaired due to limitations on pot life.
On the other hand, external crosslinking agents are often incorporated into adhesives for the purposes of improving cohesive force, securing anchoring to base materials, etc. As the crosslinking agents are generally used water-soluble melamines, emulsion type isocyanates, aziridine compounds, epoxy compounds, or the like. However, the incorporation of such a crosslinking agent sometimes poses problems in production steps due to the necessity of installation of a two-liquid mixer and limitations on adhesive pot life. Furthermore, when such an adhesive is applied to a polyethylene base material and dried, there are cases where the heat resistance of the polyethylene base material imposes limitations on the drying temperature to pose a problem that the crosslinking reaction is incomplete.